1. Field of the Invention
The present invention relates to an optical coupler for branching or coupling optical signals, a beam splitter for branching optical signals, and an AWG (Arrayed Waveguide Grating type optical wavelength division multiplexer) for combining or branching optical signals. Particularly, the present invention is concerned with an optical coupler, a beam splitter, and an AWG, of a low loss type.
2. Description of Related Art
With an increase of the information transmission volume, a WDM (Wavelength Division Multiplexing) transmission system capable of remarkably increasing the transmission capacity is in wide commercial use. In this WDM transmission system, optical signals of plural wavelengths are transmitted using a single optical fiber. Therefore, optical wavelength division multiplexers are needed at input and output portions of the optical fiber which serves as a transmission path. The optical wavelength division multiplexer combines optical signals of plural wavelengths for transmitting them with use of a single optical fiber or branches wavelength-division-multiplexed optical signals transmitted using a single optical fiber into respective wavelengths. An AWG (Arrayed Waveguide Grating type optical wavelength division multiplexer) can perform optical wavelength division multiplex/demultiplex at wavelength intervals of the order of several tens of GHz to 100 GHz. For this reason the AWG is in wide use as a key device in a WDM transmission system.
In the AWG, wavelength-division-multiplexed optical signals are inputted at an equal phase to arrayed waveguides by means of a first slab waveguide. The thus-divided optical signals of the same phase are given a phase difference by being propagated through the arrayed waveguides of different optical path lengths. Next, the optical signal thus given a phase difference are inputted to a second slab waveguide. Within the second slab waveguide, light condensing positions of the optical signals are different wavelength by wavelength. Therefore, the optical signals are divided into respective wavelengths and then outputted by means of output waveguides disposed at light condensing positions of corresponding wavelengths. It is important for the AWG, as a passive part, to be as low as possible in its optical signal propagation loss. Thus, the attainment of a low loss is demanded.
One cause of loss peculiar to the AWG is that, when optical signals are propagated from the slab waveguide to the arrayed waveguides, a portion thereof leaks from between adjacent arrayed waveguides and is not incident on the arrayed waveguides. In the connections between the arrayed waveguides and the slab waveguide there occurs a loss of 1 to 2 dB (decibel) due to such leakage of light. Methods for diminishing this loss have heretofore been proposed, for example the method disclosed in Japanese Unexamined Patent Publication No. 2000-147283 (paragraph 0013, FIG. 1). In this method, as the first conventional example, tapered waveguides which is inclined in a tapered manner are formed at the position where light is inputted from the slab waveguide to the arrayed waveguides.
FIG. 1 shows a principal portion of an AWG used in this first conventional example. Wedge-like tapered waveguides 13 as buried layers are formed around connections between a slab waveguide 11 and plural arrayed waveguides 121, 122, . . . , 125, . . . in the AWG indicated at 10. The tapered waveguides 13 are formed by etching. In the connections of the arrayed waveguides 121, 122, . . . , 125, . . . to the slab waveguide 11, the height of the tapered waveguides 13 is almost equal to that of the slab waveguide 11 and becomes smaller with separation from the slab waveguide 11.
Consequently, in the connections between the slab waveguide 11 and the arrayed waveguides 121, 122, . . . , 125, . . . in the AWG 10, an electromagnetic field distribution changes gradually. Therefore, optical signals having been propagated through the slab waveguide 11 are incident without leakage on the connections with the arrayed waveguides 121, 122, . . . , 125, . . . As a result, it is possible to attain a low loss. Reversibly, optical signals having been propagated through the arrayed waveguides 121, 122, . . . , 125, . . . can be incident on the slab waveguide 11 at a low loss.
However, in forming the wedge-like tapered waveguides 13, it is necessary that an inclination be formed by changing the etched depth (height) gradually. Consequently, there arises the problem that it is necessary to use such a special photomask or etching method as causes a continuous change of light quantity. Thus, an obstacle is encountered in the productivity of the tapered waveguides 13. Further, the light propagation characteristic greatly changes depending on the thickness of the tapered waveguides 13 formed among the arrayed waveguides 121, 122, . . . , 125, . . . . It is therefore necessary to control the inclination of each tapered waveguide 13 with a high accuracy. Thus, a problem is encountered in point of reproducibility and uniformity of the shape of the tapered waveguides 13.
In an effort to solve this problem there has been proposed such a second method as in for example Japanese Unexamined Patent Publication No. Hei 10(1998)-274719 (see paragraphs 0010 and 0012, FIGS. 1 and 2) in which mesh-like waveguides are formed in connections between a slab waveguide and arrayed waveguides to diminish an insertion loss.
FIG. 2 shows, for reference, a slab waveguide and the vicinity thereof in an ordinary AWG, while FIG. 3 shows a slab waveguide and the vicinity thereof in the second conventional example referred to above. In the ordinary AWG 20 shown in FIG. 2, input waveguides 22 and output waveguides 23 are simply connected to a slab waveguide 21.
On the otherhand, in the AWG 30 of the second conventional example shown in FIG. 3, input waveguides 32 are not specially different from that shown in FIG. 2, output waveguides 33 have a transition region 34 in the vicinity of their connections with a slab waveguide 31. The transition region 34 is formed by plural waveguide paths 35 extending across the output waveguides 33. Although in the figure the waveguide paths 35 are shown in a reduced number,actually 20 to 40, preferably about 30, waveguide paths 35 are present. The waveguide paths 35 are formed of the same material as the material of the output waveguides 33. Therefore, the waveguide paths 35 can be manufactured in the same step as the step of manufacturing the slab waveguide 31 and the output waveguides 33 which constitute the AWG 30. The waveguide paths 35 are gradually smaller in width with separation from the slab waveguide 31. This is because with separation from the slab waveguide 31 the leakage of optical signals among the output waveguides 33 decreases gradually, and is to decrease the leakage through the waveguide paths 35 of optical signals under propagation through the output waveguides 33. Consequently, optical signals under propagation among the output waveguides 33 can be picked up into the output waveguides 33 effectively by the waveguide paths 35. In the AWG 30 of this second conventional example, the insertion loss can be decreased by the presence of the transition region 34. As an example, in a typical star coupler, the insertion loss can be decreased from about 0.8 dB to about 0.3 dB.
However, in the AWG 30 of the second conventional example, periodic changes in refractive index occur in the connections between the output waveguides 33 and the waveguide paths 35 extending across the output waveguides. As a result, optical signals of a specific wavelength are accumulated and appear as reflection. This reflection causes ripple, cross talk, and distortion, in the optical wavelength division multiplex/demultiplex characteristic of the optical wavelength division multiplexer and thus exerts a bad influence thereon.